1. Field of the Invention
The present invention relates to optical communications, and, in particular, to the tuning of pre-distortion for analog lasers used in optical communication systems.
2. Description of the Related Art
Some fiber optic communication systems, such as those used to distribute cable television signals, rely on analog lasers to convert electrical signals into analog optical signals for transmission over fiber optic cables. When used to transmit cable TV signals, each optical fiber carries optical signals for different television channels, where each television channel has its own unique carrier frequency.
When two (or more) analog optical signals having different carrier frequencies travel along a single optical fiber, interference between the optical signals will result. For example, when a 75-MHz signal and a 50-MHz signal travel along a single optical fiber, interference between the two signals leads to noise at (75-50) or 25 MHz and at (75+50) or 125 MHz. In fiber optical communication systems, it is known to pre-distort the electrical signals prior to conversion into optical signals by an analog laser in an attempt to correct for non-linearities in the laser. This pre-distortion essentially adds "negative interference" to the electrical signals to reduce the adverse effects of interference that occurs during the transmission of the corresponding optical signals over optical fibers. In a typical implementation, a circuit board having a special pre-distortion circuit, is placed between the source of the electrical signal and the analog laser to pre-distort the electrical signals before they are converted into optical signals by the laser.
Although a given set of analog lasers may share the same design, variations during the manufacturing process may result in differences in the certain operating characteristics from laser to laser. These differences usually imply that each analog laser will require a different pre-distortion function to achieve optimal linearity. In a typical implementation, an analog laser and a pre-distortion board are placed in a test configuration in which potentiometers on the pre-distortion board are manually adjusted to change certain capacitance and/or resistance values within the pre-distortion circuit to tune the pre-distortion board to optimize the linearity of the analog laser. A typical tuning sequence involves the application of signals corresponding to two or more different frequency channels to characterize the performance of the analog laser and adjust the potentiometer settings on the pre-distortion board for each different channel. After tuning is completed, the analog laser and the corresponding tuned pre-distortion board may then be configured together in an actual communications system.
Conventional test configurations rely on a radio frequency (RF) spectrum analyzer to generate graphical displays representing the performance of the analog laser during the tuning of the pre-distortion board. Typical graphical displays are cartesian graphs showing signal amplitude as a function of signal frequency as presented on the cathode ray tube (CRT) display of the RF spectrum analyzer. For a given channel with a particular carrier frequency, the test operator uses the graphical display to track the performance of the analog laser as the appropriate potentiometer settings are adjusted. Due both to the small size of the CRT display of an RF spectrum analyzer and to the nature of the graphical display presented thereon, the accurate tuning of pre-distortion boards for analog lasers can be both difficult and inefficient using conventional test configurations.